The present invention relates to paper feeders for computer-driven printers, and the like, and, more particularly, to high speed paper advancing apparatus for moving paper on a line-by-line basis from a supply input between the platen and printing mechanism in a printer printing lines of text and including a platen and a printing mechanism disposed adjacent the platen comprising, powered lower drive means disposed below the platen and printing mechanism for receiving the paper from the supply input and for forming a supply loop containing paper for at least one dot line feed distance between the powered lower drive means and the platen and printing mechanism; powered upper drive means disposed above the platen and printing mechanism for gripping and rapidly moving the paper the feed distance from the supply loop between the powered lower drive means and the platen and printing mechanism; and, control logic means operably connected to the powered lower drive means and the powered upper drive means for causing the lower drive means to receive the paper from the supply input and form the supply loop during the time the printing apparatus is printing a line and for causing the upper drive means to move the paper from the supply loop the feed distance when the printing apparatus has completed printing a line and requires the paper to be advanced the feed distance in order to print a next line.
Most high speed alphanumeric printers used in association with computers to be driven thereby employ so-called "fanfold" or continuous paper having removable edges on either side containing equally spaced drive holes therein. A so-called "tractor feed" mechanism is then employed to pull the paper through the printer. A typical prior art approach to paper advancing with a tractor feed mechanism is depicted in FIGS. 1 and 2.
As can be seen, a driven tractor feed 10 is placed on either side of the paper 12 so that the drive pegs 14 engaged the holes 16 in the removable edges 18 of the paper 12. The paper is fed upward vertically from a supply stack (not shown), between the print station 20 and platen 22, and over the tractor feeds 10. The print station 20 can comprise a dot matrix printhead, a shuttle printhead, a "daisywheel" printhead, or the like. The paper is typically held against the pegs 14 of the tractor feeds 10 with a spring-loaded pressure grip (not shown). Typically, one roller 24 of one of the tractor feeds 10 is connected to the platen 22 by a gear train (symbolized by the dashed line 26) to be driven in combination therewith. The other roller 28 over which the tractor belt 30 is stretched is an idler roller. The two drive rollers 24 are interconnected by a shaft. Thus, as the platen 22 is moved in the "line feed" mode, the two drive rollers 24 are rotated. This, in turn, rotates the tractor belt 30 which pulls the paper 12 up to the proper position for printing the next line of text.
Tractor feed systems such as that shown in FIGS. 1 and 2 work reasonably well for slow speed operation. As long as the sequence of operation for paper advancement as described above take place slowly enough, the starting torque requirements placed on the platen drive motor (not shown) are low and sufficient power can be developed to pull the span of paper 12 extending downward (under the effects of gravity and friction) from the tractor feeds 10 to the supply stack. In slow speed operation, there is a virtual constant "downward" pressure on all the components and clearances provided to prevent binding of parts and occurring naturally from wear to not cause problems. Likewise, the concept of "braking" and "overshoot" are meaningless. In high speed operation, however, these previously ignorable factors suddenly take on monumental proportions leading to non-operability of the paper advancing system. As can be appreciated by those skilled in the art, the paper advancing system is the potential "weak link" in a high speed printer; that is, if the paper cannot be advanced line-by-line to keep up with the printing mechanism, the printing mechanism must be slowed down below its potential.
When trying to achieve paper advance steps such as 1/72" in 1 ms or 5/72" in 2 ms as is required to meet the printing speed capabilities of certain contemporary shuttle printers, for example, the above-described tractor feed approach of pulling the paper over the platen and past the print station 20 fails dismally. If starting torques sufficient to accelerate the paper 12 to the required speed are applied, the holes 16 can be ripped out of the paper 12 as a result of the high starting inertia of the mass of paper that must be moved each time. With so-called "laser cut" paper where the lightly attached edges 18 are intended to break off smoothly, the edges 18 may just pull off and advance while the paper 12 stays put. Multi-ply paper also causes difficulties because of its mass. In those instances where the paper is brought up to speed without incident, it may not stop in time because of the same high mass (relatively speaking) in motion. Without a positive brake, the paper 12 between the print station 20 and tractor feeds 10 may overshoot slightly and then settle back down to its proper position hanging from the tractor feeds 10. This, of course, can result in a curved line of text with a high point at the beginning.
Wherefore, it is the object of the present invention to provide a paper advancing system for use in high speed printers, and the like, which can advance continuous paper on a line-by-line basis quickly, accurately, and without overshoot, or the like.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.